OBJ format: can you export faces with indexes relative to the last "o Something" (object)?
I do not understand the question. Can you specify why it is impractical for your specific case. Where and how do you plan to import the data?

Obj files will need a unique face index as different objects are separated by obj groups referring to the vertices which are included in each object. Otherwise the file would result in something different when importing somewhere else. If you want to alter the way the wavefront obj format is imported or handle the data externally in a specific way you could always parse the file for obj groups using python and sort it the way you would like for your specific application. Alternatively you could also export all objects separately using python, obviously depending on the constraints you have making it impractical for your specific case.